Depressions

These storms affect the UK throughout the year and bring wet and windy weather to the UK. Warm air (Tropical maritime) migrating north from the tropics meets cold dense air (Polar Maritime) migrating South from the Polar region. The warm air is undercut by the advancing cold air and because it has more energy and is less dense is forced to rise upwards at a COLD FRONT Ahead of this, warm air advances into cold air and is also forced to rise above this denser cold air at a WARM FRONT.At both fronts air is rising so cooling and condensation take place which eventually results in rain AT BOTH FRONTS.The rising air creates low pressure at the earth’s surface at the centre of the storm. Air rushes in from higher pressure areas around the depression giving the high winds we often associate with depressions. Eventually the cold front catches up with the warm front and an OCCLUDED FRONT is created. View an animated sequence of the life cycle of depression from the Met Office (at the bottom of the page)

The weather in a depression

Left you can see the weather a depression brings to the UK. Note that this is a cross section through the depression. As this storm passes from the West of the UK to the East what changes would occur in our weather?

Anticyclones

In contrast to depressions, anticyclones only involve one type of air mass which usually cover large areas and do not have any fronts. They are high pressure systems in which the air moves downwards towards the earth's surface. As the air descends, the molecules become compressed, the pressure increases and it warms. When air is warming, any moisture in the atmosphere is evaporated so no clouds can form. The sky is clear. Anticyclones can be very large, typically at least 3,000 km wide. Once they become established, they can give several days of settled weather. Winds are very gentle or even calm in an anticyclone, move clockwise, and this is shown on a synoptic chart by wide spaced isobars. Look here at an excellent site showing an animation of how air moves in an anticyclone

British anticyclone weatherIn Britain in summer an anticyclone will mean heat waves during the day. At night, however, as there are no clouds, heat will be quickly lost. The ground will cool sufficiently to cause condensation of water vapour in the descending warm air and mist or heavy dew may form. This will clear quickly in the morning sun. After a few days, a layer of hot air builds up at ground level, which eventually will give rise to thunderstorms, ending the anticyclone.

In winter the longer nights combined with clear skies leads to intense cooling of the land. There is an increased risk of dew, frost and thicker, more extensive fog patches which may be slow to clear or even persist.Under very calm conditions, both frost and fog may persist for several days. An anticyclone's very stable conditions and little air movement means that pollution is trapped at low levels, resulting in very poor air quality such as smogs.

The movement of air from areas of high pressure to areas of low pressure